Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional beamline ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. Energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Beamline ion implanters are typically designed for efficient operation at relatively high implant energies and may not function efficiently at the low energies required for shallow junction implantation.
Plasma doping systems have been studied for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen, which functions as a cathode and is located in a process chamber. An ionizable process gas containing the desired dopant material is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implant energies can be achieved. Plasma doping systems are described, for example, in U.S. Pat. No. 5,354,381, issued Oct. 11, 1994 to Sheng; U.S. Pat. No. 6,020,592, issued Feb. 1, 2000 to Liebert et al.; and U.S. Pat. No. 6,182,604, issued Feb. 6, 2001 to Goeckner et al.
In the plasma doping systems described above, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, known as plasma immersion systems, continuous or pulsed RF energy is applied to the process chamber, thus producing a continuous plasma. At intervals, negative voltage pulses, which may be synchronized with the RF pulses, are applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer.
In prior art plasma ion implantation systems, a process gas containing the dopant material is supplied to the process chamber for ionization and acceleration of the ions thus formed into the wafer. This approach has certain disadvantages. The process gases utilized for plasma ion implantation are frequently toxic, thus requiring safety precautions which have the effect of significantly increasing the overall processing time. For example, following completion of plasma ion implantation, the process gas is pumped from the process chamber before the wafer is moved from the process chamber to the load lock in order to reduce the risk of contamination. When a new wafer is loaded into the process chamber, the process gas is introduced into the chamber after the load lock and process chamber are isolated. These steps add to the overall process time.
In addition, process gases are typically provided as compounds which include both the desired dopant material and undesired species. For example, boron may be supplied as BF3 or B2H6. In beamline ion implanters, the undesired species are removed from the ion beam by a mass analyzer. However, plasma ion implantation systems do not include a mass analyzer. Thus, undesired species, such as hydrogen or fluorine, are implanted into the wafer with the desired dopant material. The undesired species may be unacceptable in some applications.
Accordingly, there is a need for plasma ion implantation systems and methods which overcome some or all of the above disadvantages.